The present invention relates to a control valve for a variable displacement compressor employed, for example, in a vehicle air conditioner.
As shown in FIG. 9, a vehicular variable displacement compressor is provided with a displacement controlling mechanism as disclosed, for example, in Japanese Unexamined Patent Publication No. Hei 10-278567 or in Japanese Unexamined Patent Publication No. Hei 11-223179. The displacement control mechanism has a control valve for controlling the compressor displacement to maintain the discharge pressure having, which correlates with the refrigerant flow rate of a refrigerant circuit, at a target level. The valve position of the control valve is adjusted to adjust the internal pressure of the crank chamber (crank pressure). The compressor changes its displacement according to the crank pressure.
In the displacement control mechanism disclosed in Japanese Unexamined Patent Publication No. Hei 10-278567, a pressure sensor electrically detects the discharge pressure to carry out feedback control of a solenoid control valve based on the detected discharge pressure. In the displacement control mechanism shown in FIG. 9, the discharge pressure Pd is mechanically detected by the control valve CV, and the position of the valve body 101 depends on the detected discharge pressure Pd.
However, in the displacement control mechanism disclosed in Japanese Unexamined Patent Publication No. Hei 10-278567, a pressure sensor, an expensive part, is used. The pressure sensor must be wired manually, which increases the cost of the air conditioning system.
In the displacement control mechanism in FIG. 9, the valve body 101 of the control valve CV is located in a gas passage 102 connecting a discharge chamber to a crank chamber. A force is applied to the valve body 101 to open the gas passage 102 based on the discharge pressure Pd. Further, a force based on the crank pressure Pc within a valve chamber 103 acts upon the valve body 101 to close the gas passage 102. Therefore, the discharge pressure Pd and the crank pressure Pc are involved in positioning of the valve body 101. More specifically, the valve body 101 is positioned to maintain a constant pressure difference between the discharge pressure Pd and the crank pressure Pc.
For example, in the case where the crank pressure Pc is increased excessively, the displacement control mechanism increases the compressor displacement to maintain a constant pressure difference between the discharge pressure Pd and the crank pressure Pc. As a result, the actual discharge pressure Pd exceeds the target discharge pressure Pd (set) by a wide margin, which exerts excessive stress upon the compressor and the piping of the refrigerant circuit. Therefore, it is essential to reinforce the structures of the compressor, piping, etc. or to incorporate an open valve for preventing excessive increases of the discharge pressure Pd in the discharge pressure region. This increases the cost of the air conditioning system.
It is an object of the present invention to provide a control valve for a variable displacement compressor which can smoothly control the discharge pressure using no electrical constitution and which does not cause excessive increase of the discharge pressure.
To attain the above object, the present invention provides a control valve used for a variable displacement type compressor. The compressor varies the displacement in accordance with the pressure of a crank chamber. The compressor has a discharge pressure zone, the pressure of which is a discharge pressure, and a control passage, which connects the crank chamber to a zone in which the pressure is different from the pressure of the crank chamber. The control valve is located in the control passage. The control valve comprises a valve housing. A valve body adjusts the size of the opening of the control passage in accordance with the discharge pressure. The valve body is exposed to the pressure of the control passage. The valve body moves in accordance with the discharge pressure such that the displacement is varied to counter changes of the discharge pressure. The pressure in the control passage is applied to the valve body without hindering movement of the valve body due to an increase of the discharge pressure.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.